This invention relates generally to absorption refrigeration systems. More particularly, the invention relates to diagnostics for absorption refrigeration systems. More particularly still, the invention relates to a method and apparatus for performing diagnostics on a corrosion inhibitor present in absorption refrigeration systems.
There are a number of different pairs of refrigerants and absorbents that are used in absorption refrigeration systems. One such pair is water and a halogen salt of lithium, such as lithium bromide, lithium chloride or lithium iodide mixed to form a concentrated aqueous solution of the lithium salt. Another such pair is water and ammonia.
Iron and iron alloys like steel and copper and copper alloys are typical construction materials used in absorption refrigeration systems. Corrosion of these materials can cause difficulties. Not only is metal loss of concern but also the oxidation of ferrous metal can be accompanied by evolution of gaseous hydrogen. If not purged, the hydrogen can interfere with the proper operation of the system. Corrosion is of particular concern in systems that use halogen salts of lithium. And regardless of the refrigerant/absorbent pair used in a particular system, metal corrosion rates increase as system temperatures increase.
It is well known in the prior art that the addition of a salt of chromium, such as lithium chromate, to the refrigerant/absorbent solution in an absorption refrigeration system, is effective in reducing metallic corrosion. The presence of the chromate compound promotes the formation of a protective layer of iron and chromium oxides on the surfaces of the system that are in contact with the absorbent. With a decrease in iron oxidation, there is also a corresponding decrease in the production of noncondensible hydrogen. There is some concern, however, about the health risks that chromium presents. At least one government authority, the U.S. Environmental Protection Agency, has identified chromium as a carcinogen, and has prohibited the presence of chromium compounds in systems that are open to the atmosphere. Absorption refrigeration systems are, of course, closed systems, but a certain amount of working fluid from the system can become exposed to the atmosphere through the taking of samples, the manufacturing process and spills during handling and filling. And, at the end of the service life of a system, the system charge will necessarily require disposal of the working fluid, including the chromium compounds that it contains.
To address the foregoing concern, there has recently been developed a chromium-free aqueous solution, typically of a halogen salt of lithium, for use as a working fluid in an absorption refrigeration system. In addition, the solution also contains a compound containing a molybdate, a compound containing a borate and perhaps also, a compound containing a silicate. The added constituents act as effective corrosion inhibitors, with the inhibiting performance of the fluids being superior to lithium chromate inhibitors. This improved system of corrosion inhibitors is described in greater detail in U.S. Pat. No. 5,547,660 for Absorption Refrigeration System working Fluid with Molybdate, Borate, Silicate Inhibitor Blend by Downey and assigned to Carrier Corporation, which patent is incorporated herein by reference. Moreover, this improved system of corrosion inhibitors is employed in the WB-1 inhibited LiBr absorption chillers manufactured and sold by Carrier Corporation.
The aforementioned WB-1 inhibited LiBr absorption chiller of the Carrier Corporation uses an aqueous solution of water and a lithium halide, specifically lithium bromide (LiBr), as the working fluid (sometimes termed xe2x80x9cbrinexe2x80x9d), and employ a further solution of lithium molybdate (Li2MoO4), lithium borate and lithium silicate as the corrosion inhibitor. Although the lithium borate and lithium silicate inhibitors remain in solution in the aqueous working fluid in adequate quantities throughout the life of the fluid system, the same may not be so with respect to the molybdate inhibitor. The lithium molybdate is only sparingly soluble in the LiBr brine, and must be maintained in the 100-200 ppm range to assure the desired action as a corrosion inhibitor. However, during start-up and/or during other times of stress on the refrigeration system, the molybdate inhibitor may become sufficiently depleted as to take it below the preferred concentration range and thus expose the system to corrosion problems.
To minimize the risk of corrosion problems which may result from an insufficient concentration of the molybdate inhibitor, it has been the practice to obtain samples of the LiBr brine in the field at the site of the absorption refrigeration system and to then send them to another location for analysis. That analysis is typically performed by a non-portable, relatively expensive technique and equipment, such as inductively coupled plasma-atomic emission spectroscopy (ICP-AES). This process occasions undesirable delays (measured in days) and significant monetary costs. Although various types of on-site analyzers and analysis techniques have been employed for monitoring the level of chromate inhibitor concentrations, including color comparators and spectrometers, those techniques as they presently exist are not suitable for determining molybdate concentrations. Similarly, existing analytical processes for determining concentrations of molybdate in refrigeration systems are operative if the working fluid is water, but not if it contains a lithium halide brine, such as lithium bromide.
Accordingly, there is a need for determining molybdate inhibitor concentrations in lithium halide brines using a method and/or apparatus which facilitates on-site analysis. The method and apparatus for making such on-site analysis should be relatively portable and economical, and provide rapid and accurate determination of molybdate inhibitor concentration.
The present invention provides a method and apparatus for making a rapid and accurate analysis of the concentration of molybdate inhibitor in a lithium halide brine at the site of a refrigeration system containing the brine. The analysis may be performed using relatively standard, portable equipment in a non-complex manner.
According to the invention, there is provided a method and apparatus for quantitatively analyzing molybdate inhibitor in a lithium halide brine, including lithium bromide and/or lithium chloride, typically at the site of a refrigeration system containing the brine. The system comprises reagent means and a means for determining molybdate concentration based on color analysis. The reagent means is mixed with a sample of the lithium halide brine having an unknown concentration of molybdate inhibitor, and is selected to create a characteristic color of the sample caused by the presence of the molybdate inhibitor, the intensity of that color also being a function of the concentration of molybdate inhibitor. The reagent means may be an acidified reducing agent or a separate acid and reducing agent, selected to produce a stable color change, and does so through a pH adjustment and a redox reaction. A spectrophotometer responsive to the optical wavelength of the characteristic color of the sample is a preferred means for indicating the presence of molybdate inhibitor in the brine sample. The spectrophotometer is also responsive to the intensity of that color to provide an indication of the concentration of the molybdate inhibitor in the sample. A color comparator provides an alternate means for determining the molybdate concentration.
In a representative refrigeration system in which the brine is lithium bromide, the selected reagent is stannous chloride Sn(II)Cl2 in hydrochloric acid HCl (), and the resulting characteristic color of the sample containing the molybdate inhibitor absorbs visible light in the range of 520-580 nm, and more particularly is a pink color which absorbs visible light with a maximum near about 550-560 nm. The spectrophotometer may be a hand-held colorimeter operative only near about 550 nm.
The process of the invention includes obtaining a sample of the lithium halide brine containing the unknown quantity of molybdate inhibitor, mixing a selected reagent with that sample to provide a characteristic color, analyzing the sample to determine the presence and optical intensity of that characteristic color, and providing an indication of the concentration of molybdate inhibitor in the brine sample as a function of the determined presence and intensity of the characteristic color.
The analysis of the sample is conducted spectrophotometrically, preferably using a portable spectrophotometer or colorimeter responsive to the optical wavelength of the characteristic color. The spectrophotometer is field calibrated using a brine sample having a known concentration of molybdate inhibitor, typically zero, to establish a reference. For a lithium bromide brine, the selected reagent is stannous chloride in hydrochloric acid, and the resulting characteristic color corresponds to a wavelength in the range of 520-580 nm, typically pink with a maximum near 550-560 nm.